But speaking for myself, as a Game Design major who took a fair bit of classes on the subject, it is just very 'meh'. You took a 6, copied it twice, and rotated it about an axis to form a crude triangle. The sixes themselves are very bland, no attention grabbing font or typeset was used; just boring, fat, rounded sixes. They're not incorporated into any larger design or motif, there is no attempt to play off of more well known iconography, such as the pentagram. Those who are interested in being provocative would find it too tame, and those not so inclined would find it too uninteresting. It's not fun, it's not a parody, it's not terribly creative or thought provoking. I'm sure that, unless it was pointed out, most passerby's wouldn't even discern that it was just a trio of rotated sixes, or intuit that the number of the beast was the point rather than it being just another dumb corporate logo shirt (maybe for a skateboard gear manufacturer, or a tech startup). I mean, it's just black text on a white shirt; and even here, the idea of inverting the colors (black shirt with a white design) and making use of negative space, appears to have been a design decision too far to bridge.

So, in my educated opinion, it's a very lackluster and mediocre design. Even if I were interested in having a 'in your face' number of the beast shirt, the design fails at being evocative. If it's trying to be clever, it's simply not. And if it's trying to be subtle, it succeeds, if only because it's so subtle as to be utterly forgettable.

I mean, I'd much rather wear this...

It's minimalist, but also making use of negative space and color. It's not even on matte black, but rather the black is subtly meshed with dark grey, giving it a bit of texture. It's the mark of the Doom Slayer (the titular space marine protagonist from the DOOM reboot last year), and also doubles as a cool piece of subversive gamer gear. It's a potential conversation starter, and anyone that just 'gets it' is most likely just another gamer; more likely to elicit a knowing nod and 'cool shirt' than anything else. The design itself is an original piece of art, and wearing it is done in appreciation of the subject it appeared in, the aforementioned stellar DOOM (2016) reboot.

^ The Doom Slayer in his space marine armor. The mark in red appears just above the helmet visor. ^

To add, if I wanted to do a creative number of the beast design, I'd do one incorporated into a large bar-code. There was a rather hysterical 'satanic panic' at the introduction of bar-code scanners in certain parts of the United States, with fundies claiming that soon they'd be bar-coding people and that it was the mark of the beast!

So a black shirt with a large generic bar-code in white, while making use of negative space, it also clues in the viewer that this is not just your typical bar-code and that something else is afoot. Below the bar-code, where typically the product code would be, is where you would get creative with hiding your 666. Either by making it super obvious by just using sixes. Personally I'd option for something more subversive, like displaying 666 in binary, which looks like 00110110 00110110 00110110.